Description
We present an organoid regeneration assay in which freshly dissociated human mammary epithelial cells from healthy donors are grown in adherent/rigid or floating/compliant collagen I gels. In both conditions, luminal progenitors (CD49f+EpCAM+) form spheres, whereas basal cells (CD49fhiEpCAM-) generate branched ductal structures. However, in compliant but not rigid collagen gels, branching ducts form alveoli at their tips, express basal and luminal markers at correct positions and display contractility, which is required for alveologenesis. Thereby, branched structures generated in compliant collagen gels resemble terminal ductal-lobular units (TDLUs), the functional units of the mammary gland.